Hi there Barbara mfromdenver,
If you're not seeing the other person's video during FaceTime calls on your iMac, you'll want to start by making sure the software on your computer is updated to the most current version available, which is macOS Ventura 13.5. You can see the currently installed version by going to the Apple menu () > About This Mac. If needed, learn how to update your Mac here: Update macOS on Mac - Apple Support
Next, check your network connection and work through the suggestions below to see if they help:
If you need help with your connection or call quality
A slow or busy Wi-Fi network might cause issues with your FaceTime call. If you experience connection alerts, or low-quality audio or video, or an exclamation point
over a video tile:
* Make sure that you and your recipient have a fast Wi-Fi or cellular connection. FaceTime calls over Wi-Fi require a broadband connection.
* Check for issues that might be slowing down your connection, such as someone streaming content over your network.
* If you have questions about your internet speed, contact your internet service provider.
You'll find this information and more steps to try in this support article: If FaceTime isn't working on your Mac - Apple Support
Take care.